remove readable stream API, make synchronous, remove multibuffer detecti...

i was trying to pipe some streams1 streams to this and they were emitting `.readable` events, so then I decided this module should get simpler + doesn't need to have stream knowledge at all. so now you can just give it a buffer and it will return either an error or the successfully detected type

I also removed the multibuffer detection because we probably won't be using that in dat

also maybe you can think about renaming this module to something else... maybe `guess-buffer-format`?